Exhaust gas emissions from traffic, particularly road traffic, constitute a significant part of harmful emissions caused by human activity to the environment. For example in 1998, exhaust gas emissions from traffic made up about 50% of all emissions caused by the combustion of fossil fuels to the environment in Finland.
Certain upper limits are set by legislation for harmful exhaust gas emissions of vehicles used in road traffic, such as for the emissions of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx) and fine particles. These limit values may vary according to the vehicle (type of vehicle and engine, age of vehicle) and, to some extent, also according to the country. The trend is to continuously and gradually reduce said limit values as well as to bring them to a uniform international level. Authorities setting limit values for exhaust gas emissions include, for example, the Commission of the European Union and the United States Environmental Protection Agency (US-EPA).
According to regulations addressed directly to vehicle manufacturers, new vehicles entering the market must fulfil the criteria for exhaust gas emissions which are valid at the time. The meeting of said criteria is controlled by means of various type approval tests. The type approval tests include, for example, the measurement of exhaust gases during a specific simulated test period including accelerations, decelerations and stops when the vehicle is driven in a dynamometer. However, the type approval tests are only made for a number of vehicles before the approval of said vehicle type/model for sale and use, wherein they can naturally not be used to monitor the condition of vehicles which have already been taken into use.
Of vehicles in actual use in road traffic, significant differences can be found in exhaust gas emissions between different vehicles, due to e.g. the condition and/or age of the vehicles. Older vehicles and/or vehicles in poorer condition cause considerably higher emissions than newer vehicles which are technically more developed and in good condition. Therefore, it can be stated that as the engine technology of vehicles and the technology in the reduction of exhaust gases are developed further and as the vehicle stock is simultaneously renewed, fewer and fewer old vehicles and/or vehicles in poor condition will cause a significant part of all emissions caused by vehicles. Investigators in the field have presented estimates that for example at present, about 10% of the vehicles cause about 50% of the CO emissions from all the vehicles.
By reason of what has been presented above, the authorities should control the observation of statutory limit values also vehicle by vehicle when the vehicles are already in use in road traffic. Typically, such a control is made in connection with the statutory inspections of vehicles at regular intervals. In an inspection situation, as well as in maintenance, exhaust gas emissions are typically measured during idle running or fast idle running, without substantially loading the engine. Consequently, this measuring method does not correspond to the normal use of the vehicle in road traffic, with a varying load on the engine. Moreover, in several countries, for example in Finland, regular annual inspections of vehicles are only started after several years of the introduction of a new vehicle. Therefore, the vehicle may, even before the first inspection or during the interval between the annual inspections, cause emissions which exceed the limit values but which are not detected by the authorities.
Consequently, new measuring and monitoring methods are needed to monitor efficiently the exhaust gas emissions of single vehicles in a situation corresponding to their normal use and irrespective of inspection times or other predetermined times of testing. Various solutions are known from prior art to use road-side measurement stations to determine emissions caused by single vehicles passing the measurement station, by remote measuring techniques directly when the vehicles are moving.
U.S. Pat. No. 5,498,872 (Stedman et al.) presents a solution for the remote measurement of exhaust gases from a moving vehicle. In this method, infrared (IR) and ultraviolet (UV) radiation is directed through an exhaust gas plume emitted by a vehicle after the vehicle has passed the measurement station. Means for emitting and detecting IR and UV radiation of the measurement station are placed on different sides of the lane along the road. The concentrations of said exhaust gases are determined on the basis of the absorption caused by the exhaust gases contained in the emissions at the wavelength band specific to each component. The measurement of CO and HC concentrations is based on the absorption in the IR range, and the NO concentration is measured by means of the absorption in the UV range.
U.S. Pat. No. 5,831,267 (Jack et al.) presents a method which largely corresponds to said U.S. Pat. No. 5,498,872 but in which the measurement of the NO concentration is implemented in the IR range, wherein the apparatus becomes simpler, because a separate light source in the UV range will thus not be needed.
In view of the present invention, said U.S. Pat. No. 5,831,267 (Jack et al.) can be considered to represent the closest prior art.
The measurement of exhaust gas emissions by the above-mentioned methods based on the absorption in the measuring beam is a demanding task, because one of the most important basic principles of absorption spectroscopy is not fulfilled: the exact length of absorption range is not known, because the precise shape of the exhaust gas plume is not known and also the shape of the exhaust gas plume will vary quickly in time by the effect of e.g. turbulence and wind, as the concentrations of the exhaust gas plume are simultaneously diluted.
The use of absorption spectroscopy in measurements of gas concentration is based on the known law of Lambert and Beer:I(λ)=I0(λ)e−k(λ)x  (1)
In Formula 1, I(λ) is the intensity of a measuring beam which has passed the gas layer to be measured in an absorption length x, and I0(λ) is the original intensity of the measuring beam before the absorption caused by the gas layer. The factor k(λ)[1/m] is an absorption coefficient which depends on the effective absorption cross-section Q(λ)[m2] as well as on the gas concentration N [1/m3] according to Formula 2.k(λ)=Q(λ)N  (2)
In a situation, in which the effective absorption cross-section Q(λ) and the absorption length x of the gas are known, it is possible to determine the gas concentration N by means of the ratio I/I0. The effective absorption cross-section Q(λ) can be determined by calibration measurements made in advance, and/or its value for the gas in question can be found out in prior art.
Below, the term transmission will be used for the ratio I/I0, which may thus receive values between zero and one.
In the above-mentioned solutions of prior art, Stedman et al. and Jack et al. base their measurements on the presumption that the distributions and dilutions of CO2 and the other emission gases CO, HC, NOx contained in the exhaust gas are similar. Thus, by measuring the transmission of the actual emission gases CO, HC, NOx and simultaneously the transmission of CO2 in the IR range, it is possible to determine, with high precision, the concentration of each emission gas in relation to the concentration of CO2.
Furthermore, Stedman et al. and Jack et al. presume that the concentration of CO2 in the exhaust gas is known and substantially constant (independent of the driving situation), wherein by means of the constant concentration determined for CO2 it is also possible to determine the absolute concentrations of the other measured emission gases by means of the transmissions proportioned to the transmission of CO2.
To determine the concentration of CO2 in the exhaust gas required in said method, Stedman et al. and Jack et al. solve the stoichiometric combustion equations. However, this approach involves a number of various problems.
The first problem is that, to solve the combustion equations, it is necessary to make presuppositions about the ratios between hydrogen and carbon contained in the fuel and in the exhaust gases. Stedman et al. suppose that the ratio between hydrogen and carbon in both is 2:1. Jack et al. suppose that the ratio is 1.85:1 in fuel and 2.33:1 in exhaust gas. Although the presupposition of Jack et al. is more realistic, it does not take into account fuels with different compositions, such as for example petrol and diesel oil, and thereby the different compositions of the exhaust gases.
Another problem of this approach is that the combustion in the engines does not, by any means, always take place stoichiometrically. Nonstoichiometric combustion may occur in petrol engines for example in situations in which the engine and/or the fuel supply is defective, or they have been intentionally modified, for example for a higher power output. Petrol engines may thus operate with a too rich or too thin mixture with respect to a stoichiometric mixture of fuel and air. Because of their operating principle, diesel engines also normally operate with an excess of air; in other words, there is always an excess of air in relation to the fuel in the cylinder with respect to the concentration required for stoichiometric combustion.
The third problem is that the above-mentioned approach completely disregards chemical reactions taking place in a catalytic converter which is possibly used in the vehicle to change the composition of the exhaust gas.
The above description can thus be summarized by stating that to determine the absolute concentration of CO2 in exhaust gases, it will not be sufficient to solve the stoichiometric combustion equations in general, which will give, as the result, a constant value for the concentration of CO2 in the exhaust gases, irrespective of the driving situation. If the CO2 concentration determined in this way is used further to determine other emission gases by proportioning the transmissions measured for them with the transmission of CO2, inaccuracies will also be caused in the determination of the concentrations of said other emission gases.
U.S. Pat. No. 5,583,765 (Kleehammer) presents a remote measuring technique intended particularly for heavy vehicles, to determine the total weight and exhaust gas emissions of a single vehicle, such as a trailer lorry, within allowed limit values. The measurement apparatus, which is preferably set up by the side of a road in a region where there is an upgrade, makes it possible to collect following information about a single vehicle and the measurement situation/site:                model/type data of the vehicle (dead weight, engine type, transmission properties) for example by means of a bar code sign placed in the vehicle for this purpose,        speed of the vehicle for example by means of a radar speedmeter,        ambient conditions at the measurement site, such as for example air temperature, wind velocity, relative humidity, and air pressure,        temperature of the exhaust gas plume for example by means of an IR camera,        upslope of the road (gradient of the ascent) at the measurement site,        chemical composition of the exhaust gas plume measured by spectroscopy,        registration data of the vehicle, for example, by automatically identifying the numbers on the registration plate from an image which is taken of the vehicle.        
The method of Kleehammer is essentially based on the presupposition that the temperature of the exhaust gas plume correlates with the power output of the engine. In other words, the higher the power output of the engine, the higher the temperature of the exhaust gas plume. In the method, the vehicle is identified, and the speed of the vehicle and the temperature of the exhaust gas plume are determined when the vehicle is driven uphill, wherein said speed and temperature can be used to obtain information about the power output of the engine of the vehicle. This information can further be used to determine the total weight and load of the vehicle when the properties of the vehicle type in question (dead weight, engine type, power train), the slope of the hill and the other ambient conditions (air temperature, wind velocity, relative humidity, air pressure) are known.
In the method of Kleehammer, the chemical composition of the exhaust gas plume is also measured spectroscopically. Further, this measured profile of the exhaust gas emissions is compared with a reference profile which can be determined when the vehicle type and the power output of its engine in the measuring situation are known. In U.S. Pat. No. 5,583,765, Kleehammer briefly mentions (e.g. column 5, line 60 to column 6, line 4) that the reference profiles of exhaust gas emissions can be based, for example, on the limit values for the vehicle type in question, or on measurement data produced for the vehicle type by vehicle manufacturers or independent testing agencies. Furthermore, in U.S. Pat. No. 5,583,765, Kleehammer does not describe in more detail, how the spectroscopic measurement of the chemical composition of the exhaust gas emissions is implemented.
In view of the present invention, it is essential that the measurement of the profile of exhaust gas emissions and the determination of the reference profile according to the Kleehammer method as presented in U.S. Pat. No. 5,583,765 are carried out as completely separate operations and the profiles produced by them are only compared with each other to determine, whether the exhaust gas emissions (measured profile) of the vehicle in question fall within the allowed limits. The information contained in the reference profile is not utilized in any way to improve the accuracy of the measuring result.
As the Kleehammer method is substantially based on the presupposition that the temperature of the exhaust gas plume correlates with the power output of the engine, the method is primarily suitable for heavy diesel-engined vehicles only, which discharge an exhaust gas plume that is sufficiently large and hot for accurate measurement of the temperature of the exhaust gases. In vehicles equipped with a catalytic converter, such as petrol-driven cars, the temperature of exhaust gases discharged from the exhaust pipe into the air does not correspond directly to the power output of the engine any longer but is primarily dependent on the operating temperature of the catalytic converter itself.
The primary aim of the present invention is to provide a new method for determining and controlling exhaust gas emissions from a moving vehicle or a corresponding object by a remote measuring technique, which method makes it possible to measure the absolute concentrations of emission gases contained in the exhaust gas plume with a significantly higher precision than in solutions of prior art.
It is also an aim of the invention to provide a measuring system implementing the aforementioned method.
In the solution according to the invention, the vehicle under examination and its model/type are identified, and further, the driving situation of the vehicle is determined. Thus, by means of modeling, it is possible to determine the exhaust gas emissions, and particularly the calculatory CO2 concentration of the exhaust gases, from the vehicle, particularly in the driving situation in question.
The basic idea of the invention is that no predetermined constant value is presupposed for the CO2 concentration of the exhaust gases, but the estimate for the CO2 concentration to be determined for the vehicle by calculation may vary according to the vehicle in question and the driving situation at the moment of measurement.
The calculatory value for the CO2 concentration, determined in this way and being more precise than in prior art, further makes it possible to determine the actual harmful emission gases, such as CO, HC, NO, more precisely by absorption spectroscopy and by proportioning the transmissions measured for said other emission gases with the transmission of CO2. In other words, the method according to the invention solves the problem caused by the fact that the precise absorption length and/or the precise shape of the exhaust gas plume and/or the dilution of the exhaust gases are not known. When the absolute concentration of CO2 is known, it is possible to determine, on the basis of the optical transmission measured for CO2, the so-called effective absorption length which is the same for all the emission gases and which can thus be used to determine the absolute concentrations of the emission gases by means of the optical transmissions measured for them.
In an advantageous embodiment of the invention, the optical absorptions caused by all the emission gases to be examined, as well as by CO2 which is used as an indicator, are measured by measuring all said gases substantially at the same point in the exhaust gas plume and also by storing the measuring signals substantially at the same time. This will improve the accuracy of the measurement, because the composition and shape of the exhaust gas plume will thus be the same for all the gases to be measured.
In an advantageous embodiment of the invention, also the NO measurement is performed in the IR range, wherein the measuring system becomes simpler, because there will thus be no need for a separate illuminator in the UV range, optics and a detector. A known problem with NO measurement in the IR range is the strong absorption spectrum of water vapour falling in the same wavelength range and tending to interfere with the NO measurement. According to the invention, this problem is avoided by performing the NO measurement in the IR range by using a so-called correlation technique using an optical filter or filter arrangement as a wavelength filter in the reference and/or measuring channel, wherein the transmission as a function of the wavelength correlates with the comb-like structure of the spectral lines of the absorption spectrum in the neighbourhood of 5.25 μm of NO. Because of better selectivity which is thus achieved in the optical measurement, the effect of water vapour on the result of the NO measurement is thus effectively minimized, and the sensitivity and accuracy of the measurement are improved. Preferably, said filter means used is either a Fabry-Perot comb filter or a so-called NO gas cell filter.
The most important advantages of the present invention include a significant improvement in the accuracy of the measurement of emission gases when compared with solutions of prior art. This makes it possible to classify vehicles, for which the measurements have been taken, as passing or violating the criteria set for exhaust gas emissions, with a smaller margin or error.
The invention improves the accuracy of measurement particularly for vehicles equipped with a catalytic converter and for diesel vehicles.
Furthermore, since the model and state of motion of the vehicle in the measuring situation and under the measuring conditions are known according to the invention, it is possible to use a calculatory vehicle model to determine the real fuel consumption of the vehicle at the moment of measurement, wherein it is further possible to determine the coefficients of emissions of harmful gases for the vehicle, in the form of g/km (and/or g/s and/or g/kWh). The emission coefficients obtainable in this form can be used to evaluate the total emissions caused by the vehicles, and they make it possible to make a direct comparison, for example, with results relating to the quality of air in urban areas. If the fuel consumption of the vehicle is not known, then it is not possible to convert the emission gas concentrations obtained by the absorption measurement to said emission coefficients in a realiable manner.
The following more detailed description of the invention to be explicated with examples will more clearly illustrate, for anyone skilled in the art, advantageous embodiments of the invention as well as advantages to be achieved with the invention in relation to prior art.
The appended drawings are only intended to illustrate the invention, and thus the structures and components shown in them are not drawn to correspond to their dimensions in reality. For the sake of clarity, the figures do not show components and/or functions which are irrelevant and obvious for anyone skilled in the art as such.